Sand consolidation is a well known term applying to procedures routinely practiced in the commercial production of petroleum, whereby wells are treated in order to reduce a problem generally referred to as unconsolidated sand production. When wells are completed in petroleum-containing formations which formations also contain unconsolidated granular mineral material such as sand or gravel, production of fluids from the formation causes the flow of the particulate matter into the wellbore, which often leads to any of several difficult and expensive problems. Sometimes a well is said to "sand up", meaning the lower portion of the production well becomes filled with sand, after which further production of fluid from the formation becomes difficult or impossible. In other instances, sand production along with the fluid results in passage of granular mineral material into the pump and associated hardware of the producing well, which causes accelerated wear of the mechanical components of the producing oil well. Sustained production of sand sometimes forms a cavity in the formation which collapses and destroys the well. All of these problems are known to exist and many methods have been disclosed in the prior art and applied in oil fields in order to reduce or eliminate production of unconsolidated sand from a petroleum formation during the course of oil production.
The above-described problem and potential solutions to the problem have been the subject of extensive research by the petroleum industry in the hope of developing techniques which minimize or eliminate the movement of sand particles into the producing well and associated equipment during the course of producing fluids from the formation. One such general approach suggested in the prior art involves treating the porous, unconsolidated mass sand around the wellbore in order to cement the loose sand grains together, thereby forming a permeable consolidated sand mass which will allow production of fluids but which will restrain the movement of sand particles into the wellbore. The objective of such procedures is to create a permeable barrier or sieve adjacent to the perforations or other openings in the well casing which establish communication between the production formation and the production tubing, which restrains the flow of loose particulate mineral matter such as sand. Another approach involves removing a portion of the formation around the well and packing specially prepared resin-coated granular material into the formation around the wellbore which is subsequently caused to be cemented together.
It is a primary objective of any operable sand consolidation method that a barrier be formed around the wellbore which restrains the movement of sand particles into the well while offering little or no restriction to the flow of fluids, particularly oil, from the formation into the wellbore where it can be pumped to the surface of the earth.
Another very important quality of a satisfactory sand consolidation method is durability of the permeable barrier formed around the wellbore. Once the barrier is formed and the well is placed on production, there will be a substantial continuing flow of fluids through the flow channels within the permeable barrier, and it is important that the barrier last for a significant period of time, e.g. several months and preferably years, without excessive abrasive wear or other deterioration of the consolidation matrix which would cause the particulate matter to once again flow into the wellbore. This is a particularly difficult objective to accomplish in the instance of sand consolidation procedures applied to wells completed in formations subjected to steam flooding or other thermal recovery methods. The production of fluids in steam flooding operations involve higher temperatures and higher pH fluids than are normally encountered in ordinary primary production, and this greatly aggravates the stability problem of sand consolidation procedures.
An antithetical problem has developed in some of the modern sand consolidation processes. After a year or more of oil production, plugging of the consolidated sand mass filter often occurs and it then becomes necessary to workover the well, removing the plugged consolidated sand mass and then forming a new sand consolidating mass in its place. Some resin-consolidated sand filters are so durable that removal of the consolidated sand mass during workover is costly and time-consuming. Accordingly, there is an unfulfilled need for sand consolidation methods which are durable for a reasonable period of time, in the order of one or two years of production, but which are easy and inexpensive to remove when it becomes necessary to work over the well. Ideally, there is a need for sand consolidation methods which restrain the movement of sand during production for a year or more, but which are then more likely to disintegrate than to become plugged.
It is also important that the material injected into the formation should be essentially unreactive during the period it is inside the wellbore, i.e. while it is being pumped down the well and positioned where it is desired adjacent to the perforations of the production casing. It is this desire to delay the consolidation reaction that has lead to multi-step procedures in which first a catalyst is injected into the formation, after which the polymerizable resin-containing fluid is injected separately. While this reduces the propensity for the fluid to polymerize in the injection string, it does give rise to several problems which constitute inherent weaknesses in many prior art methods for accomplishing sand consolidation. First, each separate injection step increases the time and cost associated with the well treatment by which sand consolidation is accomplished. Second, injection of catalyst into the formation in advance of the polymerizable fluid does not accomplish uniform mixing of catalyst with all of the subsequently-injected polymerizable fluid to the degree necessary to ensure optimum polymerization of the resin, and thus often fails to achieve maximum, uniform strength and durability of the consolidated mass. Use of aqueous fluids to inject catalyst often gives rise to the need for yet additional preliminary steps to clean the sand to remove formation petroleum so the catalyst will be absorbed by the sand and later mix with the subsequently injected resin containing fluid.
Many materials have been utilized for consolidating sand in the formation adjacent to production of wellbores. One of the more successful agents utilized for this purpose are resins comprising oligomers of furfural alcohol which can be polymerized in situ to form a solid matrix which binds the sand grains together, while at the same time offering superior resistance to high temperatures and to caustic substances which may be encountered in steam flood operations. One of the problems in utilizing furfural alcohol oligomers to polymerize in the formation for the purpose of consolidating sand grains is failing to achieve uniform catalysis of the polymerization. Many catalysts that are effective for polymerizing furfural alcohol resins cannot be admixed with the furfural alcohol to permit a single fluid containing both the resin and the catalyst to be injected into the formation, because the time of polymerization is so short or unpredictable that there is excessive danger that the resin will polymerize in the injection wellbore. In my U.S. Pat. No. 4,427,069 there is disclosed a procedure for consolidating sand in a formation adjacent to a wellbore using an oligomer of furfural alcohol, in which the catalyst used is a water soluble acidic salt, preferably zirconyl chloride, which is injected in an aqueous solution into the formation prior to the resin containing fluid injection. The salt absorbs on the sand grains, and sufficient acidic salt remains adsorbed on the sand grain during the subsequent resin fluid injection stage that adequate polymerization occurs. Although this has been very effective in most difficult situations where sand consolidation procedures are utilized, particularly in connection with thermal flooding such as steam injection procedures, the procedure nevertheless requires a multi-fluid injection procedure which requires more time and is more expensive than is desired. Usually a preliminary sand cleaning step is required before injecting the aqueous-catalyst solution in order to remove the naturally-occurring oil film from the sand grains to ensure good catalyst adsorption on the sand. Also, although catalyst mixes with the subsequently injected polymer to a limited degree, usually sufficient to cause polymerization, it is believed that superior performance would result if the catalyst resin mixture can be made more homogenous prior to polymerization, in order to achieve a dense strong durable consolidation mass.
In U.S. Pat. No. 4,669,543 which issued June 2, 1987, there is described a method for consolidating sand using an acid curable resin and utilizing as a catalyst, the reaction product of an acid and an alkyl metal or ammonium molybdate. In that instance, the catalyst is incorporated in an aqueous carrier fluid which comprises the continuous phase of an emulsion in which the polymerizable resin is the dispersed or discontinuous phase. Thus this process requires that the emulsion be resolved or broken after it is located in the portion of the formation where the permeable consolidating mass is desired, which is difficult to achieve to the degree of completion and accuracy of timing necessary to accomplish the desired strong durable consolidating matrix necessary for a long lasting sand consolidation process.
In our copending application Ser. No. 07/223,822 we disclosed sand consolidation methods using an oligomer of furfural alcohol, a hydrolyzable ester as combination diluent and water extractor, and an oil soluble acid catalyst. While this produces a durable, temperature resistant permeable consolidated sand sieve, it has sometimes been difficult to remove during workover.
In view of the foregoing review of the current state of the art, it can be appreciated that there is still a substantial unfulfilled need for sand consolidation processes employing a polymerizable material in which complete mixing between the catalyst and the resin is accomplished prior to the polymerization reaction, in order to ensure that the polymerization reaction proceeds to completion, thus ensuring that the resultant polymer matrix posses the maximum possible strength and durability for the desired time period, but which either self destructs after a period of time or which is easily removed during workover. There is also a need for a sand consolidation process in which the number of separate fluid injection stages is reduced to a minimum of one or two, in order to reduce the time and cost of the sand consolidation method.